Kriegsmarine short signal systems--and how Bletchley Park exploited them

Cryptologia, Jan 1999 by Erskine, Ralph

Like most of BP's Enigma operations, Hut 8's work ultimately depended entirely upon the skill of the men and women in the British naval intercept stations. A single misheard, badly written or mistranscribed letter in an intercept could lead to disaster in a bombe menu. If, in consequence, Shark traffic was not solved or was badly delayed, men and vital ships would be lost if the OIC could not re-route a convoy past a waiting U-boat pack. Important traffic therefore sometimes had to be doubly, and even quadruply, banked by assigning additional radio sets and operators to intercept it.

SHARK B BAR SIGNALS AFTER 10 MARCH 1943

The third edition of the Wetterkurzschlussel took effect on 10 March 1943.81 Since Hut 8 did not have a copy of the new codebook, the weather short signals could no longer be used against Shark. On 8 March, the Director of Naval Intelligence had warned the Admiralty, presumably on BP's advice, that the change would blind BP against Shark for "some considerable period, perhaps extending to months."82 However, Hut 8 had studied other methods of attack, and had used some B bar signals as links in menus mainly derived from weather short signals. On 19 March, Sir Stewart Menzies, the head of the British Secret Service, who was also the Director of BP, triumphantly advised Prime Minister Winston Churchill that "We have been successful in reading the U-boat telegrams (sic) for the 16, 17 and 18 instant." Churchill replied "Congratulate y[ou]r splendid hens!"83 With the help of B bar signals,84 Hut 8 broke 87 days of Shark traffic between 10 March and 30 June 1943. To do so, it had to integrate all the information available to determine which U-boat had sent a signal, its position and the probable type of signal. The most important sources of intelligence were shore HF-DF and other Shark decrypts, especially instructions to send short signals. Radio fingerprinting (RFP - the electronic characteristics of individual wireless transmitters)85 and Tina (the morse "hand" of individual U-boat operators).86 The reported positions from convoys and sightings by RAF Coastal Command aircraft were also employed, but were not so helpful.

From 10 March until 20 May 1943 (four days before BdU withdrew the defeated U-boats from the North Atlantic), about 10 B bar signals were sent daily, with as many as 70 being transmitted on one day. After 20 May, they dropped to about six daily, mostly from the Bay of Biscay.

Since signals giving the remaining fuel in tonnes or cubic metres were encoded in two distinct groups (UGIU and HKUX), Hut 8 would have had to run different bombe menus for such signals.

Reports from boats returning home could also be useful. BdU's SWOs required boats to give 36 or 48 hours' notice of arrival (group "UKUK" or "UKVL", respectively) at Brest, Lorient and St. Nazaire - the time at which they would rendezvous with their escort vessel.90 The U-boats had to give notice for several reasons. Submarine chasers secured the entrance channels against British submarines by starting a sonar (Asdic) sweep around the meeting place. If British submarines were thought to be present, the chasers would then escort the U-boats within the 50 meter limit. Minesweepers always escorted the boats in shallow waters near river mouths within the 25 meter limit because of the danger from British mines.91 But since rendezvous were generally within the period from two hours before, to two hours after, high water in order to minimise the danger from mines, Hut 8 could guess the specific group, while the rendezvous signals were distinguished by their transmission frequency, and length and time of transmission, and with the help of shore HF-DF. Two-group signals from similar positions tended to include a request for beacon signals from a shore homing transmitter, such as "ULMD" ( "Request homing signals from No. 1 beacon, Lorient" ).92